Abstract The public's level of mental health literacy remains low, despite growing access to information regarding mental illnesses. Because few measures exist to assess the level of knowledge of mental illnesses in lay samples, the Multiple-Choice Knowledge of Mental Illnesses Test (MC-KOMIT) was developed, initially for use in a study involving police officers, some of whom received 40 h of training focused on recognizing mental illnesses, using verbal de-escalation techniques, and making mental health referrals when appropriate. This report details the initial development of the 100 questions in the item bank and the eventual selection of the final 33 items that were retained. For these 33 items, internal consistency was demonstrated, and test-retest reliability was examined among officers not in the mental health-related training who completed the test on Monday morning and again on Friday afternoon. Construct validity was examined by three hypothesis tests: the MC-KOMIT was sensitive to change related to the educational intervention, scores were significantly correlated with years of educational attainment, and officers reporting past or current treatment for a mental health problem scored higher than those without such a history. This initial report suggests that the MC-KOMIT may be useful in quantifying knowledge of mental illnesses in police officers and other diverse lay samples.